Photolytic C-O Bond Cleavage with Quantum Dots

Article abstract of DOI:10.1021/acs.chemmater.9b00943

Quantum dots are used as photoredox catalysts to drive bond cleavage in lignin model substrates. Cadmium selenide quantum dots selectively cleave C-O bonds with yields comparable to the best transition-metal-based molecular photocatalysts, thereby emulating depolymerization of the β-O-4 linkages that account for 45-60% of all linkers in native lignin. Compared to their molecular catalyst counterparts, the quantum dots demonstrate higher turnover frequencies, higher surface tunability for solvent versatility, and lower catalyst loading under mild, ambient temperature reaction conditions. The robust nature and solvent versatility of these quantum dot photoredox catalysts enable a direct, single-vessel route to convert benzylic alcohols (a majority of species in native and processed lignin) into high-value guaiacols and acetophenones without any prerequisite filtration, purification, or solvent change.

Full text of DOI:10.1021/acs.chemmater.9b00943

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Article
Photolytic C—O Bond Cleavage with Quantum Dots
Michael J. Enright, Kito Gilbert-Bass, Harrison Sarsito, and Brandi M. Cossairt
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b00943 • Publication Date (Web): 11 Mar 2019
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Photolytic C—O Bond Cleavage with Quantum Dots
Michael J. Enright, Kito Gilbert-Bass, Harrison Sarsito, and Brandi M. Cossairt *
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Department of Chemistry, University of Washington, Seattle, Washington, 98195, United States
A variety of approaches have been explored to isolate these
ABSTRACT: Quantum dots are used as photoredox catalysts
to drive bond cleavage in lignin model substrates. Cadmium
selenide quantum dots are used to selectively cleave C—O
bonds with yields comparable to the best transition metal-
based molecular photocatalysts, thereby emulating depoly-
merization of the β-O-4 linkages that account for 45-60% of all
linkers in native lignin. Compared to their molecular catalyst
counterparts, the quantum dots demonstrate higher turnover
frequencies, surface tunability for solvent versatility, and lower
catalyst loading under mild, ambient temperature reaction
conditions. The robust nature and solvent versatility of this
quantum dot photoredox catalysis enables a direct, single ves-
sel route to conversion of benzylic alcohols (a majority species
of native and processed lignin) to high value guaiacols and ace-
tophenones without any prerequisite filtration, purification, or
solvent change.
high value components by targeting cleavage of the abundant
β-O-4 linkages, which account for 45-60% of all linkages in the
complex lignin polymer backbone.24,25 Cleavage of Cα-O bonds
has been successfully accomplished via reductive, oxidative,
and redox-neutral pathways for both model substrates and in
larger lignin bioframeworks.26–35 However, these processes of-
ten require an over pressure of hydrogen, temperatures of 80
°C, or expensive transition metal catalysts. One series of studies
of notable interest uses a visible light absorbing iridium cata-
lyst, [Ir(ppy)2(dtbbpy)]PF6, to selectively drive C-O bond cleav-
age at ambient temperature and pressure.26,35–37 It is important
to note that the β-O-4 linkages are composed of benzylic alco-
hols in native lignin, as seen in Figure 1. Lignin depolymeriza-
tion often first requires oxidation of benzylic alcohols prior to
photochemical reduction, thereby requiring a multi-step route
to depolymerization products that involves purification, filtra-
tion, and solvent transfer between each step.26,38,39
INTRODUCTION
Photocatalysis has re-emerged at the forefront of synthetic
methodology development as a powerful means of activating a
wide range of bond making and bond breaking transformations
in organic chemistry. In particular, photoredox catalysis has ex-
perienced a recent expansion beyond the traditional area of so-
lar fuels into the realm of organic synthesis.1 While metal
polypyridyl complexes and organic dyes have dominated pho-
toredox platform development, quantum dots have recently
emerged as excellent candidates for coupling light and organic
synthesis.1–5 Quantum dots, are excellent candidate photocata-
lysts due to their high photostability, large extinction coeffi-
cients (>106), tunable optoelectrical properties, and customiza-
ble surface chemistry. For these reasons, quantum dots have
been used extensively as photocatalysts in hydrogen evolution
and related solar fuels applications.6–14 However, they remain
relatively limited with respect to exploration across the vast
landscape of light-catalyzed reactions.4,15–19
One such untapped application is biomass conversion. Bio-
mass is a globally abundant, renewable carbon source and is
widely considered the most sustainable carbon alternative to
petroleum.20,21 A primary component of dry plant matter (lig-
nocellulosic biomass) is lignin, an amorphous polymer that
lends structural integrity to plants. Lignin accounts for 15-35%
of the mass of plant matter but 40% of the energy content. Fur-
thermore, lignin accounts for 30% of non-fossil organic carbon
and is the only major biopolymer heavily composed of aromatic
groups that could serve as an ideal renewable source of high
value aromatic commodity chemicals (such as coumaryl, co-
niferyl, and sinapyl alcohols) if these high value units could be
apportioned from the parent structure.22,23
Figure 1. (Top) The molecular structure of lignin from a spruce
tree40 with highlighted examples of model benzylic alcohol sub-
strates explored in this study (red) embedded in the biopoly-
mer backbone and other β-O-4 linkages (pink) indicated. Soft
woods, such as spruce, typically have a lower content of β-O-4
linkages than other plant matter. (Bottom) General scheme of
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oxidation and photochemical reduction of a lignin model sub-
strate from a benzylic alcohol (BA) to guaiacol and 4’-methox-
yacetophenone via a benzylic ketone (BK) intermediate.
filtered through Celite and concentrated using the rotary evap-
orator in a bath of room temperature water. The crude product
is a yellow/white solid. Prior results have isolated BK by chro-
matography on SiO2 (70:30, hexanes/EtOAc). We were able to
isolate purified BK by suspending the crude product in 70:30,
hexanes/EtOAc, vigorous stirring and centrifugation to sepa-
rate purified product from the colored supernatant with yields
of up to 85%. BK could also be recrystalized in hexanes or
70:30 hexane/EtOAc.
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As reaction conditions are developed for improved biomass
degradation, it is important to consider quantum dots as viable
photoredox catalysts for these processes. In comparison to mo-
lecular light harvesters and traditional transition metal-based
photocatalysts, quantum quantum dots have demonstrated
longer lived excited states, greater photostability, increased
photon absorbance per molecule, and broader absorption
spectra.41,42 Furthermore, both the electronic properties and
redox potentials of quantum dots can be finely tuned by modi-
fying the size and surface chemistry of the nanocrystal.43 More-
over, the increased surface area of a quantum dot when com-
pared to a molecular catalyst would translate to an increased
probability of catalyst-substrate encounters, thereby enhanc-
ing charge-transfer and turnover rate. Quantum dots have been
used previously in combination with lignocellulose, which
served as an electron reservoir for hydrogen evolution in acidic
media.44
Synthesis of benzylic alcohol model lignin substrate
(BA). The synthesis of 2-(2-methoxyphenoxy)-1-(4-methoxy-
phenyl)ethanol (BA) was accomplished similarly to prior re-
ports.26,45 A 100 mL round bottom flask was charged with BK
(1.7 g, 6.2 mmol), THF (28 mL), and water (7 mL). Sodium bo-
rohydride (0.47 g, 12.4 mmol) was slowly added portion-wise
over 3-5 minutes to maintain a gentle evolution of gas in the
reaction vessel. After bubbling ceased, the reaction mixture
was stirred for 3 h at room temperature. The reaction was ulti-
mately quenched with saturated aqueous NH4Cl solution (50
mL) before dilution into water. The aqueous portion was sub-
sequently extracted with Et2O (3 x 50 mL). All of the combined
organic extracts were washed twice with brine, dried over
MgSO4, filtered, and concentrated in vacuo. Prior reports puri-
fied by chromatography on SiO2 (75:25, hexanes/EtOAc), how-
ever, we were able to isolate purified BA via recrystallization
in a minimum amount of 75:25, hexanes/EtOAc in yields of up
to 90%.
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In this work, we demonstrate the versatility of quantum dots
as photocatalysts in Cα-O bond cleavage using lignin model sub-
strates. Compared to the best transition metal molecular cata-
lysts for these applications, quantum dots demonstrated im-
proved turnover frequency with lower catalyst loading across
several common organic solvents. Furthermore, when using
quantum dots as photocatalysts, lignin model substrates, such
as 2-(2-methoxyphenoxy)-1-(4-methoxyphenyl)ethanol (BA),
could be taken from benzylic alcohols to cleavage products, 4’-
methoxyacetophenone and guaiacol, in a single vessel without
any purification, isolation, filtration, or solvent exchange re-
quired. This improved performance is observed for both cad-
mium selenide quantum dots (CdSe QDs) with native, long-
chain oleate or phosphonate ligands in dichloromethane, and
following ligand exchange in acetonitrile.
Reductive C–O Bond Cleavage of BK. This procedure was
modified to suit quantum dots as the photoredox catalysts from
prior reports.26,36 A 1-dram vial with a small magnetic stir bar
was loaded with 2-(2-methoxyphenoxy)-1-(4-methoxy-
phenyl)ethanone (BK) (0.33 mmol, 1.00 equiv), solvent (DCM
or MeCN, 1.67 mL), N,N-diisopropylamine (1.0 mmol, 3.0
equiv), proton source (triethylammonium salt or formic acid,
1.0 mmol, 3.0 equiv) and catalyst (CdSe QDs or
[Ir(ppy)2(dtbbpy)]PF6). The vial was capped, and the reaction
mixture was stirred under illumination with white LEDs to give
a constant flux of 200 mW/cm2 across the entire face of the re-
action vessel. Extractions between 50-100 μL were made over
the course of the reaction to track reaction rate. The solvent
was removed under vacuum and suspended in CDCl3 for evalu-
ation by NMR. See Table S1 for more information.
EXPERIMENTAL SECTION
Materials. All quantum dots were synthesized using stand-
ard Schlenk technique and ligand exchange was carried out in
a
glovebox under dry nitrogen. Cadmium oxide (CdO,
>99.99%), selenium (99.99%), 1-octadecene (ODE, 90%), oleic
acid (OA, 90%), Trioctylphosphine (TOP, 97%), 2-bromo-1-(4-
methoxyphenyl)ethanone, potassium carbonate (>99%), guai-
acol (>99%), sodium borohydride (98%), [Ir(2-phenylpyri-
dine)2(4,4’-ditertbutylbipyridine)]PF6
Single vessel C-O bond cleavage from BA. (Oxidation of BA
to BK) A 1 dram vial with a small magnetic stir bar was charged
with
2-(2-methoxyphenoxy)-1-(4-methoxy-phenyl)ethanol
(BA) (0.20 mmol, 1.00 equiv), dichloromethane (2.0 mL), silica
gel (100 wt. % of benzylic alcohol), and [4-AcNH-TEMPO]BF4
(Bobbitt’s salt, 1.05 mmol, 1.05 equiv). The vial was capped and
the heterogenous mixture was stirred at room temperature un-
der white LED illumination until all BA had been converted into
BK in 3.5 hrs. Light is not necessary to facilitate this reaction
and were illuminated for modular consistency to the reduction
from BK setup described previously. (Reduction of BK) After
3.5 hrs, N,N-diisopropylamine (0.6 mmol, 3.0 equiv), tri-
ethylammonium PF6 (0.6 mmol, 3.0 equiv), and CdSe QDs
(6.00*10-6 mmol, 3.00*10-5 equiv) dissolved in a minimum
amount of dichloromethane (0.1 mL) were quickly added to the
reaction vessel and the reaction continued under white LED il-
lumination. No filtration or purification steps were made be-
tween oxidation and reduction steps. See Table S2 for more in-
formation.
([Ir(ppy)2(dtbbpy)]PF6), N,N-diisopropylethylamine (99.5%),
[4-AcNH-TEMPO]BF4 (Bobbitt’s salt, 97%), sodium hexafluor-
ophosphate (NaPF6, 98%), sodium tetraphenylborate (NaPF6,
>99.5%), and triethylammonium chloride, were purchased
from Sigma Aldrich and used as received without further puri-
fication. Acetone, diethyl ether, tetrahydrofuran, deionized wa-
ter, dichloromethane, and acetonitrile were purchased from
various sources.
Synthesis of benzylic ketone model lignin substrate
(BK). The synthesis of 2-(2-methoxyphenoxy)-1-(4-methoxy-
phenyl)ethanone (BK) was accomplished similarly to prior re-
ports.26,45 A 500 mL round bottom flask equipped with a large
reflux condenser with flowing water was charged with 2-
bromo-1-(4-methoxyphenyl)ethanone (13.7 g, 60 mmol), po-
tassium carbonate (12.3 g, 89 mmol), guaiacol (8.2 mL, 74
mmol), and acetone (250 mL). The resulting suspension was
Synthesis of triethylammonium precursors. (TEAH PF6)
Sodium PF6 and triethylammonium chloride are each dissolved
in water in their own beakers at approximately 5 mL per 5 g of
o
stirred and heated to reflux (73 C) for 3 h in an oil bath. The
solution evolves from faint yellow to orange. After which it was
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Chemistry of Materials
solid. The two solutions are mixed together, stirred thoroughly,
and crystalized in a refrigerator. The resulting white crystals
are collected in a Buchner funnel and redissolved in a minimum
amount of water. The TEAH PF6 is recrystallized once again in
a refrigerator and collected again with a Buchner funnel. TEAH
PF6 is was placed in a round bottom flask and dried under vac-
uum and heated to 75 oC for several hours in an oil bath to re-
move residual water. It should be noted that TEAH PF6 melts
around 100 oC under vacuum. (TEAH BPh4) The BPh4 analogue
of the triethylammonium salt was prepared in a similar manner
using NaBPh4.
Yvon FluoroMax-4 fluorescence spectrophotometer. Charac-
terization by NMR spectroscopy and GC-MS was carried out us-
ing a 300 MHz Bruker Avance spectrometer and Agilent 5973
Gas Chromatograph - Mass Spectrometer respectively.
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Additional details about experimental methods and charac-
terization of reagents and catalysts is included in Tables S1 –
S3 and Figures S1 and S2 in the Supporting Information.
RESULTS AND DISCUSSION
Stephenson and co-workers report an iridium catalyst capa-
ble of Cα-O bond cleavage across a set of lignin model substrates
at room temperature within a single day.26 Most notably, they
report nearly 90% conversion of the benzylic ketone, 2-(2-
methoxyphenoxy)-1-(4-methoxyphenyl)ethenone (BK), to 4’-
methoxyacetophenone and guaiacol within 12 hrs with 1 mol
% catalyst using formic acid as a proton donor and diisopro-
pylethylamine as a hole quencher. As described below, we suc-
cessfully reproduced and exceeded these results using quan-
tum dot photocatalysts after system optimization (Table S4 and
Figures S3 – S5).
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Synthesis of CdSe QDs. The selenium precursor,
trioctylphosphine selenide, is prepared by dissolving 0.060 g of
100 mesh Se powder (99.999%) in 0.84 g of trioctylphosphine
(1.01 mL) and 4.16 mL of 1-octadecene until the solution goes
completely clear. Next the cadmium precursor is prepared by
loading 0.134 g cadmium oxide, 1.20 mL of oleic acid, and 8.00
mL of 1-octadecene into a 50 mL three-neck flask. The flask is
degassed for 30 minutes at room temperature before heating
to 240 °C under nitrogen. The cadmium precursor will turn
clear when cadmium oleate has formed. Once the cadmium so-
lution has gone completely clear, 5 mL of selenium precursor is
added rapidly. The reaction can be monitored by UV-Vis and
photoluminescence as the solution color advances from yellow
to orange to red.46 The size of the particles is controlled by
quenching the reaction (remove heat) when the target size of
the nanoparticles is achieved. Particles will reach ~3.3 nm af-
ter 4 minutes. The nanocrystals are purified by repeat dissolu-
tion/precipitation using toluene as the solvent and precipita-
tion with methanol. Typically, the first two precipitation steps
require ethyl acetate as a cosolvent to remove 1-octadecene.
See Table S3 for more information.
It is not expected that simply replacing quantum dots in a
system designed for an iridium photocatalyst would lead to the
optimal catalytic conversion. To adopt quantum dots for bio-
mass depolymerization, it is a prerequisite to ensure compati-
bility between all components (i.e. catalyst, proton source, hole
quencher, and solvent) in the photodriven reaction system for
Cα-O bond cleavage. While formic acid and ascorbic acid are
common proton donors in many photocatalytic systems, stabil-
ity challenges are often encountered in many solvents due to
the strong coordination of the conjugate bases of these acids to
the surface of the quantum dots. For example, rapid precipita-
tion of CdSe quantum dots capped with carboxylate and phos-
phonate ligands was observed upon addition of formic acid. We
discovered that triethylammonium salts (TEAH+) conveniently
serve as proton sources with no detectable perturbation of
quantum dot stability across a wide range of organic solvents.
Suitable TEAH+ salts are easily prepared, purified, and isolated
in high yields from mixing triethylammonium chloride with the
Ligand exchange to trans-4-cyanocinnamate capped
CdSe soluble in acetonitrile. This ligand exchange procedure
is adopted from Kroupa et al.47 A solution of oleate capped CdSe
QDs was prepared in dichloromethane in a nitrogen-filled
glovebox and its concentration determined using absorbance
spectroscopy.46 The conjugate acid equivalent amount of trans-
4-cyanocinnamate ligand necessary to completely exchange
the standardized QD solution (900 ligands per QD) was sus-
pend in 5:1 acetonitrile:isopropyl alcohol with gentle soni-
cation. The ligand solution was added dropwise to the QD solu-
tion with vigorous stirring, and the exchange was allowed to
proceed for 15 min. Ultimately, the QD solution volume to lig-
and solution volume was 10:1. Acetonitrile was added upon
visual precipitation of the particles to ensure the particles re-
mained suspended in the solvent mixture. The exchanged QDs
were isolated from byproduct oleic acid and excess trans-4-cy-
anocinnamic acid by first removing DCM in vacuo followed by
multiple precipitation, centrifugation and redissolution cycles
using acetonitrile as the solvent and hexanes as the antisolvent.
sodium salt of
a desired counter anion. For example,
[TEAH][BPh4] is highly soluble in acetonitrile while
[TEAH][PF6] is readily solubilized by dichloromethane. The
hole quencher, diisopropylethylamine, was found to work
without need for modification, likely due to the large steric bulk
around the Lewis basic nitrogen.
A control experiment, evaluating the use of triethylammo-
nium as a proton source, shows iridium catalyzed systems have
improved turnover rates when compared to the previously re-
ported formic acid system. In acetonitrile, direct replacement
of formic acid for [TEAH][BPh4] led to a 2.2× increase in turno-
ver frequency, while a 4.7× increase in turnover frequency was
observed with [TEAH][PF6] in dichloromethane. It should be
noted that dichloromethane had not been previously examined
as a solvent in the iridium system.
Additional apparatus details. All photocatalytic reactions
were carried out in 1 dram vials and stirred at 1500 rpm. Two
illuminated white light LEDs (Hyperikon MR16 GU10, LED
6.5W (50W Equivalent), 500 Lumen, 4000k (Daylight Glow),
CRI 84+, 120 Volt, 40° Beam Angle) were focused on each side
of the vial at a distance of 3 cm apart with the 1-dram vial posi-
tioned. This illumination provided a measured energy flux of
200 mW/cm2 across the visible spectrum between 400 and 600
nm.
In addition to substitution of the proton source, the iridium
catalyst was replaced by CdSe quantum dots to facilitate con-
version of the lignin model substrate (BK) to products. CdSe
quantum dots were readily suspended in acetonitrile following
ligand exchange with trans-4-cyanocinnamic acid.47 Conver-
sion of BK to 4’-methoxyacetophenone and guaiacol is
achieved over the same 16 hr time frame as the iridium cata-
lysts. In fact, the quantum dots required significantly less cata-
lyst to obtain the same product conversion rates. Only 0.003
mol % (333 times less) quantum dot catalyst was required to
observe near complete Cα-O bond cleavage, as shown in Table
Sample characterization. UV-visible absorption spectra
were obtained with an Agilent Cary 5000 spectrophotometer
and fluorescence spectra were recorded using a Horiba Jobin
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